Incandescent lamp filaments emit visible and non-visible radiation when an electric current of sufficient magnitude is passed through the filament. A substantial amount of energy radiated by an incandescent lamp filament, however, is in the form of non-visible radiation such as infrared radiation. As a consequence, the radiative efficiency of a typical tungsten filament, measured by the ratio of power emitted at visible wavelengths to the total radiated power over all wavelengths, is relatively low, on the order of 6 percent or less.
It has been observed that the radiative efficiency of such common filament materials as tungsten can be increased by texturing the filament surface with submicron sized features. A method of forming submicron features on the surface of a tungsten sample using a non-selective reactive ion etching technique is disclosed by H. G. Craighead, R. E. Howard, and D. M. Tennant in "Selectively Emissive Refractory Metal Surfaces," 38 Applied Physics Letters 74 (1981). Craighead et al. disclose that improved radiative efficiency results from an increase in the emissivity of visible light from the tungsten. As used herein, emissivity, is the ratio of the radiant flux, at a given wavelength, from the surface of a substance (such as tungsten) to the radiant flux emitted under the same conditions by a black body. The hypothetical black body is assumed to absorb all and reflect no radiation incident upon it.
Craighead et al. disclose that the emissivity of visible light from the textured tungsten surface was found to be twice that of a non-textured surface, and suggests that the increase is the result of a more effective coupling of the electromagnetic radiation from the textured tungsten surface to free space. The textured surface of the tungsten sample disclosed by Craighead et al. had depressions in the surface separated by columnar structures having a cross-section of approximately 0.15 micrometers (microns) and a height above the filament surface of approximately 0.3 microns.
Another method for enhancing incandescent lamp efficiency by modifying the surface of a tungsten lamp filament appears in a paper entitled "Where Will the Next Generation of Lamps Come From?", by John F. Waymouth, dated September 1989, pages 22-25 and FIG. 20, Fifth International Symposium on the Science and Technology of all Light Sources, York, England, Sep. 10-14, 1989. In this paper Waymouth hypothesizes that filament surface perforations measuring 0.35 microns across, 7 microns deep, and with walls 0.15 microns thick, would serve as waveguides which effectively couple radiation in the visible wavelengths between the tungsten and free space, but inhibit the emission of non-visible radiation from the filament. As compared to a conventional filament, the radiative efficiency of such a filament would be increased and less electrical energy would be required to produce the same lamp brightness. Waymouth discloses that the perforations on the filament could be produced by semiconductor lithographic techniques, but that the dimensions of the surface perforations are beyond current state-of-the-art capabilities.
In copending application Ser. No. 644,137, filed Jan. 22, 1991, another method of forming incandescent lamp filaments with patterned surface features of submicron-to-micron size cross section is disclosed. A filament is coated with one or more mask layers, and a selected pattern is cut into one mask layer with a laser beam. The features are formed on the filament surface by a stenciling method, where either a filament compatible material is deposited through the patterned mask layer, or the filament surface is etched through the pattern to form the submicron-to-micron sized surface features corresponding to the selected pattern.
It is an aspect of this invention to provide an incandescent lamp filament with improved emission of visible light, and having submicron-to-micron size crystallites on the surface thereof.
It is another aspect of this invention to form an incandescent lamp filament with improved emission of visible light by depositing submicron-to-micron size crystallites thereon.